up.protocol X-Up-Fail-Target

HTTP header

This request header contains the target selector that Unpoly updates when the server sends an error response.

An error response has a status code other than 2xx, but still contains HTML.

Server-side code is free to optimize its response to an error response by only rendering HTML that matches the provided selector. For example, you might prefer to not render an expensive sidebar if the sidebar is not targeted.

Example

X-Up-Target: .menu
X-Up-Fail-Target: body

Signaling failed form submissions

When submitting a form, Unpoly needs to know whether the form submission has failed (to update the form with validation errors) or succeeded (to update the [up-target] selector).

For Unpoly to be able to detect a failed form submission, the response must return a non-2xx HTTP status code. We recommend using HTTP 422 (Unprocessable Entity).

To do so in Ruby on Rails, pass a :status option to render:

class UsersController < ApplicationController

  def create
    user_params = params[:user].permit(:email, :password)
    @user = User.new(user_params)
    if @user.save
      sign_in @user
    else
      render 'form', status: :unprocessable_entity
    end
  end

end

The example uses Ruby on Rails, but works similarly with any other backend.